This invention relates to an apparatus for continuous hot dipping of metal strip.
In a continuous process for hot dipping metal strip, the amount of the molten metal which adheres to the strip is generally controlled by nozzles using an inert gas. This control is effected by regulating the pressure of the inert gas and the gap between the nozzles and the strip. When thin plating is desired, the pressure is set at a higher level and the gap is made smaller, and on the other hand, when thick plating is necessary, the pressure is set at a lower level and the gap is made smaller. In such a method for regulating the thickness of the plating, the problem of how to minimize the amplitude of the vibration of the strip at the nozzles is very important. If the amplitude of the vibration of the strip is considerable, the thickness becomes uneven, and in an extreme case the strip may come into contact with the nozzles causing them to clog. In particular, this problem is severe when thin plating is desired, since the gap between the nozzles is smaller in this situation. On the other hand, recent plating arrangements have become bigger and the line speed of the strip has become faster with speeds up to 200 to 250 m/min being used. The amplitude of the vibration of the strip generally increases in proportion to the line speed. Accordingly the necessity to reduce the vibration of the strip has become even greater.
One known method for reducing the vibration of the strip is to raise the tension of the strip at the nozzles, for example, to a level of 0.5 to 6.0 kg/mm.sup.2. However it has not been possible to adapt this method for the usual plating arrangement. The reason is as follows. Because the tension of the strip at the nozzles is regulated by a tension controller and tension bridle rolls which are disposed at the respective upstream and downstream ends of the overall apparatus, when the tension is raised to increase the tension at the nozzles to a high value, the tension at pre-treatment furnaces is also raised to the same value. In the pre-treatment furnaces, the strip is heated to a high temperature, and in some furnaces the temperature reaches a temperature of 760.degree. to 800.degree. C. which exceeds the A.sub.1 transformation point. Accordingly if the tension of the strip is raised to too high a value, the strip may be cut or shrunk widthwise. Therefore a tension of no higher than 0.3 to 0.6 kg/mm.sup.2 is generally recognized to be acceptable. For the above described reason, the tension of the strip at the nozzles, in such an apparatus must also be in the above range i.e. 0.3 to 0.6 kg/mm.sup.2. Therefore a certain degree of vibration in the strip can not be prevented, and the line speed of the strip can not be raised to a desired speed.